Life on Earth: Signpost to Life on Mars

Terby’s Layered Rocks. Layered rock outcrops are common all across Mars, and the Mars rover, Opportunity, has recently investigated some layered rocks in Meridiani Planum. Hundreds of layers are exposed in a deposit several kilometers thick within Terby. A history of events that shaped the northern Hellas region is recorded in these rocks, just waiting for a person or robot to investigate. MGS MOC Release No. MOC2-665, 14 March 2004Image Credit: Mars Global Surveyor, Malin Space Systems

Carol Stoker is the principal investigator for the Mars Analog Research and Technology Experiment (MARTE. MARTE has just begun its second field season drilling into the subsurface near the headwaters of the Río Tinto in Spain, searching for novel forms of microbial life. In a four-part interview with Astrobiology Magazine Managing Editor Henry Bortman, conducted just before Stoker left for Spain, she explained what MARTE hopes to accomplish. In this second part, Stoker talks about some of the problems that occurred during the first field season and explains the relevance of the MARTE project to the search for life on Mars.

Astrobiology Magazine: Last year, your team was successful at drilling into the subsurface at Río Tinto. You were able to extract drill cores, and to culture bacteria from the material you extracted. What additional science are you hoping to do in this second field season?

Carol Stoker (CS): Before we drilled last year, we had done an analysis to figure out where the water table was likely to be. We had selected 5 locations to drill. For each site we had picked, we mapped the subsurface water table, and found very specifically the depth that water appeared to be at. Our assumption was that we wouldn’t see biology until we actually hit the water table. And it turned at that the site that we thought would yield our highest science value had the water table at 100 meters (about 330 feet), so we knew we had to drill at least 100 meters at that location to have a probability of finding life. The problem was that that was expensive, and so it was also going to be our most expensive hole.

So we took a gamble and went to that site first, before we had really perfected all our procedures. Only one person on our team, Todd Stevens, had ever done microbiological drilling before. He had done it a lot and knew a lot about it, but all the rest of us were rookies. So we knew that we might make some mistakes.

As it turned out, we thought what we had done was very successful, but when we went back and worked through all the logic of what we had found and what more we would like to know, what questions reviewers might ask, we realized that we probably needed to do some stuff over again. So we’re going to go back to the same location or a nearby location and drill to about the same depth again this year.

AM: For example?

CS: As I mentioned before, one of the things that we did is culture organisms from the places where we seemed to have microbial hot spots. But the cultures were done aerobically, they were incubated in air. And then a lot of things grew up. But they were growing up in an oxygen environment. We realized: geez, we really would have liked to do those in an anaerobic environment, because we’d really like to know whether we could grow strict anaerobes [organisms that cannot surviving the presence of oxygen] from our samples.

An artist’s representation of a Mars Exploration Rover.Credit: NASA

Now obviously we had things that weren’t strict anaerobes that were growing, but that doesn’t really tell us that those are subsurface organisms that can live independently of the surface. So we’re going to start cultures in the glove box and keep them anaerobic, which we didn’t do before.

AM: Do you think that the organisms that are living down there may be completely isolated from the surface?

CS: It’s our hope. That’s actually what we have to prove in order to know that we’ve got a truly new type of subsurface biosphere.

AM: That type of biosphere has never been found before on Earth?

CS: There has been a type of subsurface biosphere identified. It’s a methanogenic biosphere and it makes a living off hydrogen that results from the abiogenic weathering of basalt. But the Río Tinto system is a different kind of system. It’s a sulfide system. We believe that it’s an iron-sulfur-based energy system, that these organisms are making a living from the oxidation of sulfide minerals. So we need to prove not only that there are organisms there. We also need to figure out their life style. We need to figure out what they’re using for an energy source, what they’re producing as byproducts. And we need to prove that their number densities are correlated with the availability of resources.

Rover computer rendering on the edge of a depression, much like Opportunity’s perch on the edge of Endurance Crater.Credit: Maas/NASA/JPL

AM: Iron and sulfur seem to have some relevance to Mars. Iron- and sulfur-bearing minerals have been identified by the Mars Exploration Rover Opportunity at several locations in Meridiani Planum.

CS: Yeah, well that’s why we picked this site, as a matter of fact.

AM: Suppose you are able to prove that there are subsurface organisms living in a completely isolated anaerobic ecosystem at Río Tinto. What would that say about the ability of similar anaerobic organisms to live below the surface in Merdiani Planum?

CS: Well, as it turns out the discovery of massive amounts of jarosite at Meridiani Planum in January of this year – and remember we started drilling in September of last year – was a big boost to our project, because we had essentially hypothesized a life style for microorganisms at this location based on the surface life style, the life style that we see living in the river water and little ponds that form as a result of little dams and whatnot. And there is this scooped out area from the crater where the mine was that’s also filled with water. All those areas have microbes living in them.

The microbes seem to be capable of living anaerobically, because if you put a dissolved oxygen meter in any of those waters, you get to a point just a few centimeters (2 or 3 inches) below the surface where you can’t measure any oxygen. The minerals in the water, in particular the iron, are really good at scavenging any oxygen. The iron comes out of the ground, gets oxidized and becomes hematite. The water is carrying a huge amount of hematite. That’s what’s giving it the red color. Below the surface the water becomes very quickly anaerobic. The bottoms of all these lakes and ponds are anaerobic. So that was one of the reasons we thought that anaerobic metabolism would be feasible.